There has been ever greater demand for energy-conserving measures in recent years as electrophotographic full-color copiers have become more widespread. In pursuit of energy-conserving measures, investigations have been carried out into technology that can cause toner to undergo fixing at lower fixation temperatures in order to lower power consumption in the fixing process. The use in toner of a resin having a sharp-melt property is preferred for improving the low-temperature fixability of toner, and in recent years, polyester resins have been used as sharp-melting resins.
For example, Patent Document 1 provides a toner composed of a high softening point polyester resin and a low softening point polyester resin in 80 to 30:20 to 70 (weight ratio). Patent Document 2 provides a toner that is prepared by using a crosslinked aliphatic alcohol-based polyester resin and a non-crosslinked aromatic alcohol-based polyester resin. Patent Document 3 provides a toner that contains a high softening point polyester with a softening point of 120 to 160° C. and a low softening point polyester with a softening point of 75 to 120° C. Patent Document 4 provides a toner that contains a polyester resin having acid value of 13 to 50 mg KOH/g and hydroxyl value of not more than 8 mg KOH/g.
These toners exhibit some effects with regard to improvement of the low-temperature fixability, but when used in high-speed machines, they provide an increased adhesive force between the fixing member and the recording paper, which can result in the recording paper wrapping onto the fixing member.
In addition, these toners exhibit reduced toner chargeability and are prone to undergo charge relaxation. In particular, when these toners are used at high print coverage rates in a high temperature/high humidity environment, toner charging level is decreased, which can ultimately produce large variations in image density and fogging in white background regions.    [Patent Document 1] Japanese Patent Application Laid-open No. H11-305486    [Patent Document 2] Japanese Patent Application Laid-open No. 2000-39738    [Patent Document 3] Japanese Patent Application Laid-open No. 2002-287427    [Patent Document 4] Japanese Patent Application Laid-open No. 2007-4149